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Okay, so in my setting I'm writing, the planet is very earth-like, similar atmosphere, liquid water exists, etc.. But the planet orbits a red giant. I've done a lot of fiddling around with various things (color of flora, etc.), but what I can't figure out is what a rainbow would look like. I've looked around quite a bit including on here, but can't seem to find an answer.

I assume that since it's not monochromatic, and we're talking about sunlight shining through water droplets, there would be still at least most of the colors of the earth rainbow visible. But would it be shifted? Wider red band or some other color? Would some colors not show up? Anyone have any ideas?

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    $\begingroup$ You're aware, are you not, that red giants are a very short-lived (cosmologically speaking) star type. They form near the end of a star's life, and don't last long enough for life to evolve on the newly-thawed ice planets that aren't swallowed up like the inner planets were... $\endgroup$
    – Zeiss Ikon
    Oct 26, 2020 at 18:53
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    $\begingroup$ One thing to consider is that rainbows partially look the way they do because of the color receptors we have. If we lacked the red color receptor, for example, we would likely see rainbows as only green and blue. Conversely, if we could see UV, we might see some of that after the blue/violet in our rainbows. If you have aliens that have different amounts or types of color receptors, that might affect how they perceive the rainbow. $\endgroup$
    – Drake P
    Oct 26, 2020 at 19:05
  • $\begingroup$ @ZeissIkon Yes, I'm aware - it's sci-fi, with humans settling on and living on the planet. Though I'd suggest that at least simple life could feasibly evolve on such a planet. But yes, I'm aware it would be hard for advanced lifeforms to evolve in the shorter-lived red giant system. $\endgroup$
    – Nathan Manning
    Oct 26, 2020 at 19:22
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    $\begingroup$ Just a note to the original poster: a red giant is about as red as an incandescent light bulb (remember them), or about as red as the modern "warm white" LED lights. The human visual system has a built-in white balance correction system, and most people have no trouble maintaing the correct white balance under incandescent or warm white light. $\endgroup$
    – AlexP
    Oct 26, 2020 at 20:33
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    $\begingroup$ Just scrap the red GIANT, and use a much more common red dwarf star. There are tens of millions times more of them, because 1) they are much likelier to form and 2) they last tens of billions of years, instead of a couple hundred thousand years like red Giants do. The rainbow will be very similar to ours, just heavier on the red. You may have visible absorption lines due to presence of chemicals like titanium oxide in the star! $\endgroup$
    – user79911
    Oct 27, 2020 at 14:37

3 Answers 3

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You'd get a rainbow with brighter red and yellow, and dimmer green and blue.

What is often missed in discussion of "red stars" (like M class red dwarves) is that the surface temperature of those stars is similar to that of the tungsten filament in an old-style incandescent light bulb -- which, though fairly yellow when compared to a truly white light source, is still "white" enough that all colors are readily visible.

You can observe a full spectrum from a tungsten lamp -- using a CD as a diffraction grating, for instance -- just as you can from the Sun, or from a blue-white class O star. The difference is how bright the spectrum is in what color, but this isn't extremely obvious until the spectrum is pretty heavily skewed, either toward red or toward blue. A so-called "red dwarf" M star would still look white to the eye (just as a tungsten lamp filament does), so the spectrum it produces, while easily distinguished from that of a hotter star with instruments, will look very much the same to the human eye.

The width of the color bands is due to the physics of refraction working with human color perception. In truth, there are no "bands" of color in the rainbow -- it's a continuous, well, spectrum. We label certain parts (red, orange, yellow, green, blue, indigo, violet) based on how we perceive color, and those colors seem to form "bands" because we perceive them that way. The wavelengths of light, however, will still be evenly spread across the width of the rainbow's band, both the width and angular size of which are determined by the refractive index and dispesion of water. As long as the water is still water (and falling rain is pretty pure, in general), you'll get the same size, shape, and width rainbow we have as long as the light is visibly white to the eye (see above).

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  • $\begingroup$ Thank you! That's about what I was thinking with the brightness. Okay, the width/'band' thing I didn't know, very interesting. $\endgroup$
    – Nathan Manning
    Oct 26, 2020 at 19:26
  • $\begingroup$ I don't like to niggle but this is an interesting question. I'd suggest that a red sun will only emit a full spectrum (i.e. like a tungsten filament) if it really is acting as a "black body", a counterexample to this is a thoriated gas mantle which radiates disproportionately strongly in the yellow-green-blue region. One thing we can say is that the width of each colour in the rainbow will be constant, even if the intensity looks wrong... to our eyes. $\endgroup$
    – Mark Morgan Lloyd
    Oct 27, 2020 at 12:10
  • $\begingroup$ Aside from the absorption lines that are used to identify elements in the star's atmosphere, stars in general are treated as black body radiators. After all, the star is mostly hydrogen, but we don't see primarily hydrogen-alpha, or all stars would be the same color of near-monochromatic red (all other hydrogen emission lines are outside our visible range). $\endgroup$
    – Zeiss Ikon
    Oct 27, 2020 at 12:53
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More reds and yellows, not as much blues and greens.

of course, its not going to be exclusively red, just brighter red. The star emits all wavelengths (of visible light at least) and as such a rainbow will have all of the wavelengths as well. Think of it like adding more red paint to a paint mixture, the other paint is still there, theres just more red now.

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The same as on our planet Earth

As other answers note, because the star is red, the red and yellow in your rainbow will be brighter than some of the other colours.

But it takes a while to travel to and from this planet. Nobody ever just drops by. So if you are on Planet Red, you are either:

  • a newcomer, and attest that the colours seem weird on this new planet,

  • or you are a baby and you do not coherently answer questions about colour,

  • or are acclimatised to Planet Red.

This means that most people on Planet Red have a colour perception that is slightly off from Terran humans. So they see the rainbow the same as we do. Their eyes (really their brains), their culture, and their languages, have all altered subtly. It's Planet Earth that will look odd.

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